About once a week, someone tells us, “The 120 volt plug on my new kiln doesn’t match my wall outlet.” Many people do not know the difference between 15 amp and 20 amp, 120 volt wall outlets, because they look almost the same. But a 20 amp, 120 volt kiln cannot be plugged into a 15 amp, 120 volt wall outlet.
The common 15 amp, 120 volt household outlet (also called a receptacle) is used throughout North America. Lamps, vacuum cleaners, refrigerators, etc. plug into it. The outlet has two vertical slots; the U-shaped hole is for the green safety grounding wire.
The 20 amp, 120 volt outlet looks like the 15 amp outlet, except one of the vertical slots is shaped like a sideways T.
NEMA, the National Electrical Manufacturers Association, has standardized these designs and designated them NEMA 5-15R and 5-20R. (The number after the hyphen indicates amperage.) Standardized plugs and outlets are so ubiquitous that we rarely think about them. But they helped to bring electricity into millions of homes and factories.